1. Field of the Invention
This invention relates generally to electrophotography and more particularly to improvements in electrostatically charging electrophotographic film with an apparatus which limits the amount of charge deposited upon the film during charging.
2. Description of the Prior Art
In electrophotography, it is common to apply uniform electrostatic charge to the surface of an electrophotographic film, which generally consists of a photoconductive layer overlying a conductive layer. The charge is then selectively dissipated in a pattern by exposing the photoconductive surface to a light image. The resulting pattern of charges produces an electrostatic latent image upon the photoconductive layer which is rendered visible by applying thereto electrostatically charged developer particles which adhere to the surface of the photoconductive layer by electrostatic forces. A permanent visible image can be obtained, for example, by using developer particles which can be heat fused to the photoconductive layer and subjecting it to a heat application step.
Charging is conventionally accomplished by exposing the surface of the photoconductive layer to a corona discharge, the polarity of which is chosen to produce the desired results upon a particular photoconductive layer being charged. Superior image reproductions are obtainable only when very uniform electrostatic charges are established on the photoconductive layer before imaging. High voltages for generating corona are particularly desirable to maintain uniformity, but can subject the photoconductive layer of the film to excessive charge buildup which can damage the photoconductive layer by current leakage into the conductive layer beneath. A number of techniques have been employed to limit the amount of charge buildup on the photoconductive layer, the most common of which is the use of electrical circuitry to limit corona production (an example being disclosed in U.S. Pat. No. 3,335,275 to King). Unfortunately, a good stable corona produced by high voltage is necessary to assure charge uniformity on the photoconductive layer. Furthermore, the common technique for limiting corona production results in an undersirable level of current leakage through the photoconductive layer.
In view of the disadvantages of the prior art, it is an object of the invention to provide means for limiting the amount of charge deposited upon the photoconductive layer of an electrophotographic film without adversely affecting corona production or excessively damaging the photoconductive layer by charge leakage therethrough.